The present invention relates to a nonvolatile semiconductor memory device and a manufacturing method thereof, in particular to a resistance random access nonvolatile semiconductor memory device and a manufacturing method thereof.
In the field of a nonvolatile memory, a flash memory, an FeRAM (Ferroelectric Random Access Memory), an MRAM (Magnetic Random Access Memory), an OUM (Ovonic Unified Memory), a PRAM (Phase change Random Access Memory; Patent Literature 1) and the like have been actively studied.
Recently, a resistance random access nonvolatile memory (ReRAM), which is different from those nonvolatile memories, has been proposed (Non-Patent Literature 1). In this resistance random access nonvolatile memory, information is written by applying a voltage pulse and varying a resistance value of a resistance change part of the memory cell. A resistance random access nonvolatile memory makes it possible to read written information nondestructively. In addition, a resistance random access nonvolatile memory has a small element area and thus is capable of being multivalued. Consequently, a resistance random access nonvolatile memory is expected to be promising as it has higher potential than existent nonvolatile memories.
Resistance change mechanisms of resistance random access nonvolatile memories are classified into two major types; an electrochemical type and a filament type. Whereas an electrochemical type requires both positive and negative voltage's for varying a resistance, a filament type makes single polarity operation possible. (Non-Patent Literatures 1 and 2)
A resistance change element has a structure formed by interposing a resistance change layer between electrodes. That means a two-terminal element. As a material of the resistance change layer of a filament type resistance change element, a transition metal oxide such as WOx (tungsten oxide), NiOx (nickel oxide), TaOx (tantalum oxide), ZrOx (zirconium oxide), HfOx (hafnium oxide), or the like is mostly used. In many cases, the initial state is in an insulated state. As a material of the electrodes, a simple substance of a metal, particularly a precious metal, such as Pt (platinum), Ru (ruthenium), W (tungsten), Al (aluminum), Cu (copper), or the like is mostly used.
FIG. 1 is a schematic view showing an example in an operation method of a typical filament type resistance change element. The resistance change element: has a structure formed by stacking an upper electrode 252, resistance change layer 241 and a lower electrode 251; and is coupled to a transistor 250 in series.
Initialization is carried out by dielectric breakdown between electrodes. That is, as shown in (a), a voltage VT.E. applied to an upper electrode 252, a voltage VB.E. applied to a lower electrode 251, and a voltage VG applied to the gate of a transistor 250 are set at 2.5 V, 0 V, and 2 V, respectively. Dielectric breakdown voltage is thus applied between the upper electrode 252 and the lower electrode 251. As a result, as shown in (b), in a part of a resistance change layer 241, a low-resistance conductive path called a filament 241a is formed like a bridge (also called “forming”) between the upper electrode 252 and the lower electrode 251. The state is called a low resistance state (LRS). On this occasion, the resistance of the filament 241a is controlled so as not to be excessively low by controlling electric current flowing in the filament 241a after dielectric breakdown through an external circuit (not shown in the figure). For example, the flowing electric current is adjusted so as to have a resistance of 1 kW.
Resistance increase is carried out by cutting a part of a filament. That is, as shown in (b), the voltage VT.E. applied to an upper electrode 252, the voltage VB.E. applied to a lower electrode 251, and the voltage VG applied to the gate of a transistor 250 are set at 1.0 V, 0 V, and 5 V, respectively. As a result, as shown in (c), a part of a filament 241a is cut (also called “reset”). This state is called a high resistance state (HRS). The cut of the filament 241a is caused by giving a power of not less than a threshold value to the filament 241a. On this Occasion, a voltage exceeding the threshold value has to be applied to both the ends of the filament 241a (P=V2/R, V>(RP)0.5). It is known that the part where the filament 241a is cut can be explained by a tunnel barrier model.
Resistance decrease is carried out by applying a voltage higher than a resistance increase voltage to a filament and thus coupling the cut filament again. That is, as shown in (c), the voltage VT.E. applied to an upper electrode 252, the voltage VB.E applied to a lower electrode 251, and the voltage VG applied to the gate of a transistor 250 are set at 2.5 V, 0 V, and 2 V, respectively. As a result, as shown in (b), the cut filament 241a is coupled again (also called “set”). The state is a low resistance state (LRS). The filament 241a is coupled again by the dielectric breakdown of a tunnel barrier.